Method and apparatus for removing electrons from CMOS sensor photodetectors

ABSTRACT

An improved CMOS sensor integrated circuit is disclosed, along with methods of making the circuit and computer readable descriptions of the circuit.

REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/029,100, filed 30 Dec. 2004 now U.S. Pat. No. 7,250,665 by inventorsZeynep Toros, Richard Mann and Selim Bencuya entitled Method andApparatus for Removing Electrons from CMOS Sensor Photodetectors.

This application is related to the commonly owned U.S. patentapplications Ser. No. 11/029,103, entitled “Method and Apparatus forVarying a CMOS Sensor Control Voltage”, by inventors Zeynep Toros,Richard Mann, Selim Bencuya, Sergi Lin and Jiafu Luo; to Ser. No.11/026,460, entitled “Method and Apparatus for Controlling ChargeTransfer in CMOS Sensors with an Implant by the Transfer Gate”, byinventors Toros et al.; to Ser. No. 11/026,582, entitled “Method andApparatus for Controlling Charge Transfer in CMOS Sensors with aTransfer Gate Work Function” by inventors Toros et al.; to Ser. No.11/026,278, entitled “Method and Apparatus for Controlling ChargeTransfer in CMOS Sensors with a Graded Transfer Gate Work Function” byinventors Toros et al.; to Ser. No. 11/029,101, entitled “Method forDesigning a CMOS Sensor”, by inventors Toros et al.; and to Ser. No.11/029,102, entitled “Method and Apparatus for Proximate CMOS Pixels”,by inventors Toros et al.; all of said applications filed on the sameday as this application and all incorporated by reference as if fullyset forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments are related to image sensors, computer readable descriptionsof image sensors, and methods for making mage sensors, and moreparticularly to such embodiments of CMOS image sensors.

2. Description of Related Art

The active pixel sensor is used in CMOS based imager arrays for avariety of applications. The sensor consists of an array of pixels(rows×columns) with the associated active circuitry on the same chip.Each pixel contains a photosensitive device that senses the incominglight and generates a ΔV difference on a floating node. The readout isaccomplished by selecting a row of pixels and reading out each column,either column by column or all columns at the same time. The XYaddressable APS is designed for CMOS technology with minor modificationsto the process for the pixel while maintaining low-power and lower costfeatures compared to the CCD technology. Another main advantage of usingCMOS process is to have the pixel array with the associated activecircuitry on the same chip and save area and cost. Despite all of thebenefits of using the CMOS process, the picture quality of the CCD imagesensors is still superior to the picture quality of the CMOS APS. One ofthe main reasons for this difference is that the CMOS process is notsuitable to designing a good pixel element, unlike the CCD process whichis designed specifically to build pixel elements that result in a highquality picture. Another limitation of the CMOS process is that theoperating voltages are low and not flexible as in the CCD technology.

SUMMARY OF THE INVENTION

This innovation describes the process methods and process integration ofan active pixel sensor that combines the advantages of both CCD and CMOStechnologies. The low noise advantages of a true correlated multiplesampling pixel (e.g., Correlated Double Sample pixel) are created in aCMOS process with low cost and high performance with minimum impact onexisting features and capabilities of the CMOS technology. Disclosedembodiments cover 4T pixel designs, although other protected embodimentscover 5T and other pixel designs.

Between sequential images, a photodetector must be cleared of allelectrons. Otherwise, image lag causes signal loss for a current imageand mixes the signals of the sequential images. Adding a shallow n-typeregion around the deeper n-type region of the photodetector guides theelectrons generated by incident photons out of the photodetector areaand towards the transfer gate, thereby reducing image lag. Bypositioning part of the pixel ring next to the shallow trench isolation,photon collection area is maximized.

One embodiment of the image sensor integrated circuit includesphotodetectors such as photodiodes, nodes such as floating diffusions,and transfer devices such as transfer gates that control a transfer ofthe electrons between a photodetector and a corresponding floatingdiffusion. Each photodetector includes 2 n-type regions, such as a deepcentral region and a peripheral shallow region. Part of the secondn-type region is between the first n-type region and an oxide structurethat isolates adjacent photodetectors from each other. The circuit alsoincludes reset devices such as reset transistors. Each floatingdiffusion node has a corresponding reset device which resets the node.The circuit also includes row and column circuitry such as row andcolumn decoders, and signal devices such as source follower and rowselector transistors.

Other embodiments include a method for fabricating the circuit and acomputer readable description of the circuit, such as a layout ortapeout.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 shows a schematic of a pixel with improved performance.

FIG. 2 shows a schematic of pixels with improved performance that sharecircuitry.

FIG. 3 shows a sketch of the potential profile in the charge collectionand transfer region.

FIG. 4 shows a sketch of charge leakage resulting from a weak barrierwhen the transfer gate is off.

FIG. 5 shows a sketch of blooming control, where the off voltage of thetransfer gate is adjustable to the desired level.

FIG. 6 shows a sketch of the transfer gate is turning on.

FIG. 7 shows a sketch of charge trapping where the transfer gate is on.

FIG. 8 shows a sketch showing that by raising the transfer gate voltage,the barrier is removed, allowing the charge to flow.

FIG. 9 shows a timing diagram of the RST operation to reset the floatingdiffusion voltage with a positive delay time.

FIG. 10 shows a timing diagram of the RST operation to reset thefloating diffusion voltage with a negative delay time.

FIG. 11 shows a timing diagram of the RO (readout) operation to read outthe charge, where the SEL (select) signal is turned low after the RST(reset) level is sampled, and the SEL signal is turned high again tosample the signal level.

FIG. 12 shows a timing diagram of the RO operation to read out thecharge, where the SEL signal is kept high until the transfer gatevoltage is turned low.

FIG. 13 is a cross-sectional schematic of a pixel with a separationbetween a typical p-well and the transfer device barrier boron p-well.

FIG. 14 is a cross-sectional schematic of a pixel with a separationbetween the transfer device barrier implant and the reset transistorp-well implant (in this example, with a separation of about 0.3 um-0.5um), and an overlap of the transfer device barrier implant with thetransfer gate (in this example, with an overlap of about 0.2 um).

FIG. 15 is a plan view schematic of a pixel with a separation betweenthe transfer device barrier implant and the reset transistor p-wellimplant, and an overlap of the transfer device barrier implant with thetransfer gate.

FIG. 16 is a plan view schematic of a pixel with an n-type diode ringimplant and showing the cross-section for the 2D simulation.

FIG. 17 illustrates part of a pixel fabrication process and shows across-sectional view of a pixel with shallow trench isolation.

FIG. 18 illustrates part of a pixel fabrication process and shows thepatterning and implantation of the transfer device barrier implant.

FIG. 19 illustrates part of a pixel fabrication process and shows thetransfer device barrier implant.

FIG. 20 illustrates part of a pixel fabrication process and shows thepatterning and implantation of a typical p-well implant.

FIG. 21 illustrates part of a pixel fabrication process and shows thep-well implant for the signal and reset transistors and isolation ofneighboring photodiodes.

FIG. 22 illustrates part of a pixel fabrication process and shows thepatterning and implantation of the photodiode ring implant.

FIG. 23 illustrates part of a pixel fabrication process and shows thephotodiode ring implant.

FIG. 24 illustrates part of a pixel fabrication process and shows thepatterning and implantation of the photodiode deep implant.

FIG. 25 illustrates part of a pixel fabrication process and shows thephotodiode deep implant.

FIG. 26 illustrates part of a pixel fabrication process and shows thepatterning and growth of the gate oxide and patterning and deposition ofthe polysilicon gates.

FIG. 27 illustrates part of a pixel fabrication process and shows thepatterning and implantation of the nldd, or floating node.

FIG. 28 illustrates part of a pixel fabrication process and shows thepatterning and implantation of the pldd, or pinning implant.

FIG. 29 illustrates part of a pixel fabrication process and showsannealing and the growth of the transistor spacers.

FIG. 30 illustrates part of a pixel fabrication process and shows thepatterning and implantation of the sources and drains of thetransistors.

FIG. 31 illustrates part of a pixel fabrication process and shows theannealing.

FIG. 32 shows a simulation of a cross-section a pixel with the resettransistor, transfer gate, and photodiode.

FIG. 33 is a cross-sectional schematic of a pixel with a p-typepolysilicon gate for the transfer gate.

FIG. 34 is a cross-sectional schematic of a photodetector of a pixelshowing the p+ epitaxial layer.

FIG. 35 is an example layout of several pixels that share circuitry suchas the floating node, reset transistor, and signal transistors.

FIG. 36 is an exemplary computer apparatus and computer code medium.

DETAILED DESCRIPTION

The pixel as illustrated in FIG. 1 is designed to overcome limitationsof the CMOS process and achieve good picture quality levels that arecomparable to CCD sensors.

The pixel consists of a pinned photodiode (PD) 210 as light sensingelement, a transfer gate (TG) 220, a floating diffusion 240 (andassociated capacitance 241), a MOSFET as reset transistor 230, a secondMOSFET as source follower 260, and a third MOSFET as row selecttransistor 270. The devices which have undergone modified fabrication130 according to some embodiments include the pinned photodiode (PD)210, the transfer gate (TG) 220, and the floating diffusion 240.

FIG. 1 also shows selectable voltage circuitry 110 coupled to thetransfer gate 220. One current-carrying terminal of the reset transistor230 is coupled to the floating diffusion 240 and anothercurrent-carrying terminal of the reset transistor 230 is coupled to acurrent-carrying terminal of the source follower 260.

The pixel is designed to be built with the CMOS process with additionalimplantation steps to improve the performance. Only modest positivebiasing voltages are required which can easily be provided by a CMOSprocess without the need for special high voltage devices. The processmodifications are in the pixel array and therefore the rest of theon-chip active circuitry need not be affected. Since the pixel uses thesame operating voltages, the readout operation, timing, digital controlblock and the analog signal chain remain the same. The pixel is a 3MOSFET+1 transfer gate pixel with a pinned photodiode as the lightsensing element.

Charge remaining in the PD from the previous frame is called lag. Lag isusually caused by incomplete charge transfer. Incomplete charge transferoccurs mostly due to one or more of:

1) Charge is trapped in the PD because of too large of a barrier betweenthe ON gate and the PD, and

2) In charge injection devices (such as CID imagers), the charge resetoccurs by turning the gates off and injecting the charge to thesubstrate. But, when the gates turn on again, some of this charge mayreturn to the potential well before recombining, resulting in image lag.

The latter cause “2)” is not the case for pixels which do not use thesubstrate for the charge reset. The PD and transfer region should bedesigned very carefully to avoid the former cause “1)”.

Two effects of lag are that charge is lost from the original framesignal (distorting the current image), and charge is added to the nextframe (distorting the next image). Therefore, image lag should beeliminated completely or at least as much as possible.

To eliminate the image lag completely, the CMOS process flow is modifiedin the pixel area. FIG. 14 illustrates the cross-sectional view of thecharge transfer in the pixel that is separated from the adjacent pixelby shallow trench isolation (STI) 702. The transfer gate 717 is a truegate structure that is placed between the pinned photodiode 712, 715 andthe n-type floating diffusion 720. The purpose of the transfer gate 717is:

a) to keep the integrated charge in the PD 712, 715 separated from thefloating diffusion 720 during charge integration period,

b) to perform complete charge transfer during charge readout by turningtransfer gate 717 on, and

c) global reset; electronically resetting the pixels by turning both thetransfer gate 717 and reset gate 716 on.

Additional implants help to perform a complete charge transfer:

1) An additional p-type implant 705, such as a lighter boron dopingimplant (also called “transfer device barrier Boron implant”).

2) A deep n-type photodiode implant 715 (e.g., low dose)

3) A shallow n-type photodiode implant 712 (e.g., ring shaped and highdose).

A fourth implant can also be used to improve performance by adjustingthe work function of the transfer device gate 717 to be “graded” fromn-type where it overlaps the sense node 720 drain to more p-type whereit overlaps the n-type photodiode 712, 715.

The high performance and low cost of this pixel innovation is alsorealized through the optimum use and placement of process layers andimplants in the baseline CMOS process. Specifically, the p-well forformation of NMOS devices in a CMOS process is employed to provideisolation between pixels. In addition, the p-well provides isolationfrom the traps and surface states associated with the STI which is usedin submicron CMOS. This isolation is used for the integrated photodiodeto achieve low dark currents more typical of a CCD device. The PLDDimplant is employed to provide junction isolation for dark currentreduction from the surface states in the diode.

The transfer device Boron implant (TDBI) helps the TG turn on moreeasily, and therefore improves the charge transfer. The deep n-implantand n-type ring implant in the PD are both used to adjust the PDcapacitance as well as charge transfer operation by introducing apotential gradient that helps the charge move towards the floatingdiffusion when the TG is turned ON.

By optimizing the pixel as described above, the amount of the chargethan can be transferred completely is maximized. Since the floatingdiffusion potential can be read out before and after charge transfer,the noise level can be reduced by correlated double sampling. Therefore,the dynamic range improves significantly, and image-lag is eliminated.The pixel described in this innovation is comparable to CCD pixels andwill result in good picture quality comparable to CCD sensors.

Advantages:

1) Reduced dark current noise: Dark current is one of the importantcontributors to the output noise. The significant component of the darkcurrent is generated at silicon/silicon dioxide interface. Pinnedphotodiode will reduce the dark current generation significantly bykeeping the surface accumulated with holes. A transfer gate with anappropriate work function, such as p-type polysilicon, reduces darkcurrent under the transfer gate and attracts holes to the region by thetransfer gate.

2) Reduced kTC noise: The design of the pixel with the transfer gateenables true correlated double sampling at the output. Therefore thenoise that is generated at the output amplifier can be eliminated. Theoutput amplifier becomes very much like a CCD sensor output.

3) Higher signal level: The additional n-type photodiode implants areintended to maximize the capacitance of the PD, and still performcomplete charge transfer during readout. The pixel ring implant evenincreases capacitance in the region between the deep implant and thenearby STI region which has been implanted with p-type dopants.

4) Increased dynamic range: Optimizing the PD capacitance and reducingthe output amplifier noise maximizes the dynamic range.

5) The same pixel can be used in shared architecture. (FIG. 2.) Sharingthe SF 260, RG 230 and Row_Sel 270 transistors increases the fill factorof the pixel. Higher dynamic range and signal-to-noise ratio areachieved.

6) A CMOS pixel sensor is designed to achieve good picture qualityassociated with CCD image sensors while maintaining

a) low-power

b) low-cost and

c) On chip active circuitry integration features of the CMOS technology.

However, these advantages are accompanied by the addition of extraimplant steps to the CMOS process to build the charge transfer device toachieve complete charge transfer.

FIG. 3-8 show the behavior of the transfer gate. The phi symbolindicates potential increasing in the downward direction of the arrow.Ideally, when the TG 220 is OFF, charge should be collected in thepinned PD 210. The difference between the full diode potential 212 andempty diode potential 211 determines the charge collection capacity ofthe pixel. FIG. 3 also shows the collected charge 214, the barrierheight 213 between the collected charge 214 and the transfer gate offpotential 226. This value can be optimized by varying the n-type PDimplant, and the TDBI implant.

If the TDBI concentration is too low, then the OFF TG 226 cannot provideenough of a barrier for electrons 214 collected in the PD 210 resultingin a constant leakage into the floating diffusion region 240. Thepotential profile of the charge leakage is shown in FIG. 4. Constantleakage is not desirable and should be avoided. The TG 220 becomesuseless and cannot isolate the charge 214 from the FD region 240 duringcharge integration. The pixel becomes like a 3 transistor pixel.Therefore, the TDBI dose and energy should be selected very carefully toavoid constant charge leakage from the PD 210 to the FD 240 to make surethat the pixel does have enough charge capacity. The dynamic range andsignal to noise ratio depend on this signal.

In case of a very bright light, the PD can get saturated and excesscharge is going to flow over the OFF TG (FIG. 5). This is not the sameas charge leakage. The TG 220 acts like a blooming control device. TheOFF gate voltage can be set to an intermediate value (shown as multiplevalues 226) during charge integration for blooming control. Excesscharge 242 is drained to the FD, and removed by turning the RG on.Setting the TG OFF voltage to an intermediate value (somewhere betweenOFF and ON states) essentially controls the signal capacity of thepixel. The highest capacity is obtained with the most negative TGvoltage. Intermediate levels reduce the signal, and therefore can beused as blooming control.

The TDBI concentration should be optimized such that while the TG isOFF, the barrier under the TG should provide enough barrier for thecharge integrated in the PD. On the other hand, when the TG turns ON,this barrier should disappear completely so that the charge can betransferred.

FIGS. 6 and 7 show the 2D potential profile of the charge transfer areawith the TG turning ON 225. Because of the overly strong TDBIconcentration, there is some charge trapped 214 in the PD 210, as shownin FIG. 7. This charge causes image lag and is not desired. The pixelshould be designed so that no charge gets trapped in the PD after chargetransfer is finished. The need to manage the charge transfer barrier atthe proper level is the most difficult aspect of the robust design ofthe charge transfer device. It is classically difficult to insure a highbarrier with no leakage in the off 226 or NO TRANSFER state whileassuring the complete transfer of a large amount of charge in the on 225or transfer state.

In our innovation, the manufacturing and performance window of operationof the pixel is increased by using elevated voltages on the transfergate. Thus even if there is some residual charge in the PD, by raisingthe TG voltage even further, this charge can be transferred from the PDto the FD (FIG. 8). Since the channel potential rises under the transfergate, the maximum absolute voltage of the transfer gate can be safelyincreased without creating a high electric field across the gate oxidedielectric materials. The use of higher voltages on the transfer gate isanticipated in the device design, resulting in a strong barrier tocharge transfer in the off state which is forced down by a largervoltage on the transfer gate during transfer. In this manner theeffective threshold of the transfer device is sufficient to blockunwanted charge transfer in the off state. The threshold of the devicewill vary in manufacture and thus the minimum gate voltage to assureadequate charge transfer will have process variation. Use of a transfergate voltage which is above the maximum required will insure that thetransfer is always complete.

The charge pump should be designed to provide at least one VT above thesupply voltage. Higher TG voltage is safe to use since this gateoperates with strong backbias. In our innovation an adjustable voltagepulse is provided to the transfer gate in which the maximum appliedvoltage and the rise and fall times of the transfer gate voltage pulsecan be adjusted. On chip adjustment through a DAC is provided to allowtesting of the charge transfer properties at a range of voltages. Inthis manner the needed manufacturing margin for complete charge transferin product operation can be verified.

After carefully optimizing the TG structure to obtain complete chargetransfer, and the PD to achieve desired signal level, the switching timeof the TG should also be considered. When the TG switches from ON toOFF, charge can spill back into the PD especially if this gate is turnedOFF too quickly. Therefore, enough time should be allowed for this gateto turn off. This time can be in the range of 50 ns to 150 ns.

While the deep n-type diode implant mainly determines the collectiondepth of the electrons, and the PD capacity, the shallower n-type ringimplant in the PD is used to increase the capacity around the edges ofthe PD. The main purpose of the shallow implant other than contributingto the PD capacity, is to provide a potential gradient toward the TG forthe electrons when this gate is turned ON. The three sides of the ringstructure neighboring the STI utilize the edges and improve the signalcapacity. The side that is adjacent to the poly-gate shifts thepotential maximum towards the FD when the TG is turned on and introducesa potential gradient from the center of the PD towards the TG edge ofthe PD, and acting like a channel for the electrons to flow from the PDover to the FD region. Otherwise, it is much more difficult to transferthe integrated charge completely to the PD, and avoid image lag. Thepotential gradient also helps the transfer time. Because of thisgradient, the electrons move faster to the floating diffusion node, andthe time for the charge transfer is reduced significantly. Sample ringimplant dose and energy are 8e11, 150 keV.

The TDBI does not extend under the TG completely. Rather, the TDBIextends, for example, by about 0.2 um. These three implants—TDBI, deepn-type diode implant, and the shallow ring-implant—and the TG length areoptimized and laid out so that whole range of the supply voltage can beused to store and transfer the charge. The threshold voltage of thisgate is reduced so that the charge transfer occurs under this gate veryclose to the surface. When the gate is turned on, the charge flows fromthe deep n-diode region to the surface where the shallow n-region is.The TDBI barrier disappears completely under the TG and the charge flowsfrom the shallow n-region into the FD.

Even though charge transfer occurs under the TG at the p-well edge veryclose to the surface, charge integration takes place in the PD with thisgate turned OFF. The collection depth is determined by the deep n-typeimplant. As more charge gets collected in the PD, the potential maximumin the silicon moves closer to silicon/dioxide interface. The deeper then-implant goes, the more the PD capacity. Deeper implants also provide amore favorable electric field implant for the collection of red light.The tail of the potential profile is also important. If the junction istoo abrupt, the collection of the electrons due to red light becomesmore difficult. Therefore, the junction depth is adjusted, for example,to about 1.5 um for this structure. While the shallow n-type ringimplant better utilizes edges of the PD, and provides a potentialgradient for the charge to transfer close to the silicon surface, thedeep n-type implant is used to have enough PD capacity and chargecollection depth. The implant dose and energy in one embodiment areabout 1e12 and 300 keV for this design.

Red light collection and overall pixel capacity are also optimized bybuilding the device on an epitaxial substrate. An P epi layer of, forexample, about 5E14 concentration with a thickness of 4 to 5 microns isoptimal in one case. The electric field from the P+ substratesconcentration of boron helps to reflect photoelectrons towards thesurface for collection by the photodiode.

For this structure, the parameters are boron with dose and energy in therange of 1.75e12 , 50 keV and 1.2e13, 200 keV. The best doping levelscan be optimized based upon consideration of additional process detailssuch as starting material doping level and the exact thermal cycles ofthe process.

Timing:

The pixel operation consists of the basic three functions:

1) Resetting the floating diffusion voltage (RST) FIGS. 9 and 10.

FIGS. 9 and 10 show a timing diagram of the RST operation to reset thefloating diffuision voltage. FIG. 9 shows the reset with a positivedelay time and FIG. 10 shows the reset with a negative delay time. Thesel voltage remains at V_(low) _(—) _(sel). The tg voltage rises fromV_(low) _(—) _(tg) to V_(high) _(—) _(tg) over a time of t_(r) _(—)_(tg), and then falls back to V_(low) _(—) _(tg) over a time of t_(f)_(—) _(tg). The rst voltage rises from V_(low) _(—) _(rst) to V_(high)_(—) _(rst) over a time of t_(r) _(—) _(rst), and then falls back toV_(low) _(—) _(rst) over a time of t_(f) _(—) _(rst).

The time t_(pw) _(—) _(rst) begins when the rst voltage begins to risefrom V_(low) _(—) _(rst) to V_(high) _(—) _(rst), and ends when the rstvoltage begins to fall from V_(high) _(—) _(rst) to V_(low) _(—) _(rst).The time t_(pw) _(—) _(tg) begins when the tg voltage begins to risefrom V_(low) _(—) _(tg) to V_(high) _(—) _(tg), and ends when the tgvoltage begins to fall from V_(high) _(—) _(tg) to V_(low) _(—) _(tg).The time t_(cov) _(—) _(tgrst) begins when the tg voltage has fallenfrom V_(high) _(—) _(tg) to V_(low) _(—) _(tg), and ends when the rstvoltage begins to fall from V_(high) _(—) _(rst) to V_(low) _(—) _(rst).

In FIG. 9, there is a positive delay time of t_(d) _(—) _(tgrst) fromthe time when the tg voltage begins to rise from V_(low) _(—) _(tg) toV_(high) _(—) _(tg), to the time when the rst voltage begins to risefrom V_(low) _(—) _(rst) to V_(high) _(—) _(rst). In FIG. 10, there is anegative delay time with a magnitude of t_(d) _(—) _(tgrst) from thetime when the rst voltage begins to rise from V_(low) _(—) _(rst) toV_(high) _(—) _(rst) to the time when the tg voltage begins to rise fromV_(low) _(—) _(tg) to V_(high) _(—) _(tg).

The time overlap of the rst voltage being at the voltage V_(high) _(—)_(rst) and the tg voltage being at the voltage V_(high) _(—) _(tg) isthe time t_(ov) _(—) _(rst) _(—) _(tg). In FIG. 9, the rst voltage risesfrom V_(low) _(—) _(rst) to V_(high) _(—) _(rst) after the tg voltagerises from V_(low) _(—) _(tg) to V_(high) _(—) _(tg),and the tg voltagefalls from V_(high) _(—) _(tg) to V_(low) _(—) _(tg) before the rstvoltage falls from V_(high) _(—) _(rst) to V_(low) _(—) _(rst). Thus inFIG. 9, the time t_(ov) _(—) _(rst) _(—) _(tg) begins after the rstvoltage has risen from V_(low) _(—) _(rst) to V_(high) _(—) _(rst) andends when the tg voltage begins to fall from V_(high) _(—) _(tg) toV_(low) _(—) _(tg). In FIG. 10, the tg voltage rises from V_(low) _(—)_(tg) to V_(high) _(—) _(tg) after the rst voltage rises from V_(low)_(—) _(rst) to V_(high) _(—) _(rst), and the tg voltage falls fromV_(high) _(—) _(tg) to V_(low) _(—) _(tg) before the voltage of rstfalls from V_(high) _(—) _(rst) to V_(low) _(—) _(rst). Thus in FIG. 10,the time t_(ov) _(—) _(rst) _(—) _(tg) begins after the tg voltage hasrisen from V_(low) _(—) _(tg) to V_(high) _(—) _(tg) and ends when thetg voltage begins to fall from V_(high) _(—) _(tg) to V_(low) _(—)_(tg).

2) Charge integration in the photodiode (INT)

3) Charge readout (RO) FIGS. 11 and 12.

FIGS. 11 and 12 show a timing diagram of the RO (readout) operation toread out the charge. In FIG. 11, the SEL (select) signal is turned lowafter the RST (reset) level is sampled, and the SEL signal is turnedhigh again to sample the signal level. In FIG. 12, the SEL signal iskept high until after the transfer gate voltage is turned low.

The rst voltage rises from V_(low) _(—) _(rst) to V_(high) _(—) _(rst)over a time of t_(r) _(—) _(rst). In response, the fd voltage rises toV_(cell) _(—) _(hi)-V_(T′) (for example, CELL_HI 250 on FIG. 1 minus athreshold voltage). The rst voltage then falls back to V_(low) _(—)_(rst) over a time of t_(f) _(—) _(rst). In response, the fd voltagefalls slightly due to gate coupling. The time t_(pw) _(—) _(rst) beginswhen the rst voltage begins to rise from V_(low) _(—) _(rst) to V_(high)_(—) _(rst), and ends when the rst voltage begins to fall from V_(high)_(—) _(rst) to V_(low) _(—) _(rst).

After a time delay of t_(gap) _(—) ₁ after the rst voltage has fallenback to V_(low) _(—) _(rst), the sel voltage begins to rise from V_(low)_(—) _(sel). The sel voltage rises from V_(low) _(—) _(sel) to V_(high)_(—) _(sel) over a time of t_(r) _(—) _(sel). In FIG. 12, the selvoltage remains at V_(high) _(—) _(sel) while the tg voltage rises fromV_(low) _(—) _(tg) to V_(high) _(—) _(tg) and falls back to V_(low) _(—)_(tg). In FIG. 12, the time t_(pw) _(—) _(sel) is the duration duringwhich the tg voltage remains at V_(high) _(—) _(sel). The sel voltagefalls from V_(high) _(—) _(sel) to V_(low) _(—) _(sel) over a time oft_(f) _(—) _(sel).

The tg voltage rises from V_(low) _(—) _(tg) to V_(high) _(—) _(tg) overa time of t_(r) _(—) _(tg). In response, the fd voltage also rises. Thetg voltage falls back to V_(low) _(—) _(tg) over a time of t_(f) _(—)_(tg). In response, the fd voltage also falls. The signal level is thedifference between this final fd voltage and the fd voltage after the fdvoltage falls due to gate coupling with the falling rst voltage.

In FIG. 11, the sel voltage does not remain at V_(high) _(—) _(sel)while the tg voltage turns high and back low. Instead, after the selvoltage has risen to V_(low) _(—) _(sel) for the first time, the selvoltage falls to V_(low) _(—) _(sel) and after the sel voltage has beenat V_(low) _(—) _(sel) for a time delay of t_(gap) _(—) ₂, the tgvoltage begins to rise from V_(low) _(—) _(tg) to V_(high) _(—) _(tg).After the tg voltage falls from V_(high) _(—) _(tg) to V_(low) _(—)_(tg) and the tg voltage has been at V_(low) _(—) _(tg) for a time delayof t_(gap) _(—) ₃, the sel voltage begins to rise for a second time fromV_(low) _(—) _(sel) to V_(high) _(—) _(sel). For both sel voltage pulsesin FIG. 11, the sel voltage rises from V_(low) _(—) _(sel) to V_(high)_(—) _(sel) over a time of t_(r) _(—) _(sel), and the sel voltage fallsfrom V_(high) _(—) _(sel) to V_(low) _(—) _(sel) over a time of t_(f)_(—) _(sel). For both sel voltage pulses in FIG. 11, the time t_(pw)_(—) _(sel), begins when the sel voltage begins to rise from V_(low)_(—) _(sel) to V_(high) _(—) _(sel), and ends when the sel voltagebegins to fall from V_(high) _(—) _(sel) to V_(low) _(—) _(sel).

Both RG and TG are turned ON to reset the PD node, and reset the PD.This ensures to remove all the residual charge from the PD (if any).FIGS. 9 and 10. Reset is followed by charge integration. Both gates arekept at low voltages while charge is integrated in the PD. At the end ofthe charge integration, charge readout period starts. This isillustrated in FIGS. 11 and 12. First, the RG is turned on and the resetlevel is sampled by turning the Row_sel transistor ON. Then the TG isturned on to transfer the charge to the FD. The Row_sel can be eitherkept at ON state, or turned ON again to sample the signal level. Thisoperation is repeated every frame time.

Design Methodology:

The pixel layout and implants in this innovation are optimized by asimulation methodology that insures a near optimal solution. Andescription of this device design flow is provided below:

Step 1: Choose a diode pinning implant. We suggest an implant similar oridentical to PLDD as the PD pinning implant.

Pldd implant can be used to pin the surface of the PD with holes to themost negative potential. This insures low dark current and a surfaceelectric field favorable to collection of blue light. Pldd is not theonly solution as pinning implant. It is preferred since it comes freewith the CMOS process and works well as the pinning implant of the PD.In this innovation the choice of the exact dose and energy for thepinning implant are less critical because the pinning implant positiongreatly reduces the effect of the electrical barrier to charge transfer.The pinning implant is self aligned to the transfer gate edge like aPLDD which reduces variation due to errors in dimensions and alignmentof the mask.

Step 2: Select a TG length depending on the pixel size, layout andarchitecture.

And select starting dose and energy implants for the Ntype Deep andNtype shallow ring implants for the photodiode.

Obtain the following by Steady-state analysis:

3) Determine the empty PD potential by adjusting the n-type deep andn-type shallow ring implants to obtain a desired amount of signalcapacity with the TG off. If it is too high, charge transfer willsuffer. If it is too low, the signal level will be too low.

And then obtain the following by transient analysis:

4) Adjust the TDBI concentration to isolate the charge from the floatingdiffusion while the TG is OFF. If there is too much leakage, increasethe TDBI doping.

5) Find the maximum signal level by overfilling the PD and reaching asteady state over time with the TG turned OFF.

6) Transfer the charge from the PD to floating diffusion by switchingthe TG ON and OFF. If charge is trapped, raise the TG ON voltage. Adjustthe ring implant dose, energy and location to achieve a potentialgradient towards the FD during charge transfer. The maximum transfergate voltage applied in the analysis is based upon the ability togenerate and manage an elevated potential in the CMOS process. Anexample is the use of a 5.5 volt maximum transfer gate voltage for a 3.3volt CMOS process. (a 3.3 volt CMOS process is a process for which a 3.3volt potential can be applied across the gate dielectric whilemaintaining acceptable long term reliability). The goal of the devicedesign is to insure that complete charge transfer occurs below thetarget maximum voltage to insure margin for manufacture.

7) If the charge cannot be transferred completely or if there is notenough diode capacity after the step, go back to the beginning andrepeat the steps, typically starting from 1) to optimize the pixel.

Optimization of the pixel is an iterative process. The convergence tothe desired solution is faster if the starting point is not very faroff. Therefore, the first guess is important. A good guess based onprevious experience makes a good starting point.

After a desired signal capacity is achieved and the pixel operation isverified by simulations, continue with the following analysis:

Color cross-talk:

8) If there is no residual charge in the PD after charge transfer, andthe desired signal level is achieved, the color cross-talk should bedetermined. For this purpose, light at different wavelengths (blue,green, and red) should be shined onto the pixel while extracting theamount of charge collected in the adjacent PD. The p-well provides verygood isolation between pixels. The TDBI function as an isolation barrierbetween pixels should be verified.

9) If the cross-talk is higher than tolerable amount, go back to earliersteps, typically from step 1),

a. adjust the depth of charge collection by changing the n-type PDimplant energy and/or

b. use p-well as oppose to the TDBI for STI, and/or

c. make the PD to PD distance larger in the layout.

Sensitivity to the misalignment of mask layers:

10) Move mask layers around to verify the critical dimensions. The pixeloperation may be very sensitive to some of the drawn locations of themask layer. Determine the most crucial layers, and the degree of thefailure if the mask is misaligned. Find more robust solutions. A clearadvantage of this innovation of the relative insensitivity of the deviceoperation to normal variation in the size and placement of the implantmasks.

Embodiments of the TDBI implant cover the left edge of the transfer gatedevice to provide an adequate barrier and isolation between the floatingnode (or sense node) and the photodiode. The TDBI concentration istargeted so that the TDBI boron under the gate can be inverted to forman N type channel. This inversion is made facile by the ability to pumpthe transfer gate to an elevated voltage to continue the charge transferprocess as the channel and photodiode potential become more positive.The TDBI implant and the deep phosphorous diode implant overlap. Thisinsures that the right portion of the transfer gate device iselectrically coupled to the photodiode.

Features for Device Performance in Integration into CMOS:

1) Use of n-type MOS Device in the Pixel:

-   -   reduces cost and assures predictable performance    -   p-well implant for CMOS n-type FET provides effective energy        barrier for electron cross-talk between pixels.    -   TDBI also provides a good barrier between the adjacent pixels.    -   Depending on the layout, both standard and lighter TDBI can be        used for isolation.    -   TDBI can also be used for reset transistor. The threshold        voltage of this transistor is reduced and the RG cam be laid out        in the same TDBI as the TG.    -   FIG. 13 sows that if the reset transistor is designed with the        p-well, the layout is optimized to integrate with the transfer        gate implant 705 insuring optimum separation 160 (˜0.3 microns        to 0.5 microns) between the higher doped NMOS p-well Edge 708        and the lower doped TDBI 705. Thus separation refers to the        implantation areas. After thermal processing steps, the        implanted dopants diffuse closer, shrinking or eliminating the        gap.    -   In our innovation adjustable voltages of increased absolute        potential are also provided for the gate of the reset        transistor. This insures that the sense node potential can be        reset to the power supply voltage of the chip to insure maximum        pixel capacity.    -   These two different devices with different functions and        required doping profiles of silicon impurities are integrated to        be in very close proximity to support pixel scaling.    -   However, in shared pixel designs this approach affords room to        use a more conventional NMOS p-well for the transfer gate        device. There is sufficient room for the NMOS device p-well edge        to be separated from the Right edge of the Transfer device.    -   The high concentration of the p-well for NMOS transistors has        decreased to within ½ order (about a factor of 3.2) of the wafer        background doping before reaching regions of TDBI doping.

2) Transfer Device Barrier Control Boron Implant

-   -   Boron implant for control of barrier potential for charge        transfer from floating diffusion node/reset node with the        following features:    -   Implant is not centered on the channel formed by the        intersection of the transfer gate poly and active but is moved        away from the n-type photodiode region. The shift is determined        by the charge transfer operation. For one embodiment, the TDBI        overlaps with the TG by 0.2 um. (FIG. 14). The top view of the        layout for the non-shared embodiment of the pixel is shown in        FIG. 15. Charge transfer occurs at the floating node edge of the        transfer gate, very close to the silicon surface. This shift        insures the best barrier properties to transfer the charge.

FIG. 14 is a cross-sectional schematic of a pixel. The pixel is formedin a <100> substrate 700 with a p-type doping of 5×10¹⁴ cm⁻³. Shallowtrench isolation 702 separates a photodiode formed from implants (N-typedeep implant 715 and N-type ring implant 712) from an adjacentphotodiode 790. There is a separation between the transfer devicebarrier implant 705 and the reset transistor p-well 716 (in thisexample, with a separation of about 0.3 um-0.5 um), and an overlap ofthe transfer device barrier implant 705 with the transfer gate 717 (inthis example, with an overlap of about 0.2 um). The transfer gate 717also overlaps the floating diffusion 720 and the photodiode. Thetransfer gate 717 is aligned with the p+ pinning implant 723. Thefloating diffusion 720 is formed to be positioned partly in the transferdevice barrier implant 705 and partly in the reset transistor p-well716. The reset gate 716 overlaps the n+ floating diffusion 720 and then+ RD 730.

FIG. 15 is a plan view schematic of a pixel. The pixel is formed with aseparation 160 between the transfer device barrier implant 125 and thereset transistor p-well implant 120, and an overlap of the transferdevice barrier implant 125 with the transfer gate 152. Also shown areNMOS transistor gates 145, photodiode area 143, and active outline 140.

-   -   TDBI doping is process dependent, for example boron with dose        and energy in the range of 1e12, 40 keV to 5e13, 250 keV.    -   The Doping level for the barrier is optimized in concert with        the implants for the photodiode n type region to insure optimum        capacity, built in anti-blooming control, and full charge        transfer to insure true CDS and low noise.

3) Ring and Core Implants for N-Type Photo Collection Region.(Photodiode Area)

-   -   The capacity and charge transfer and noise are optimized in CMOS        integration through the use of two implants to define the n-type        area and ensure optimum integration into CMOS.    -   A low dose phosphorous implant with dose in the range of 1e12 to        1e13 and energy in the range of 200 keV to 37 keV.    -   This phosphorous implant provides optimum depth for the        photodiode electric field to ensure low cross talk and high        collection of red light.    -   A second ring-shaped phosphorous implant (FIG. 16) is placed        around the perimeter of the device and in intimate contact with        the transfer channel implant to maximize the capacity of the        pixel in a CMOS context while providing a favorable electric        field. This concept is used in conjunction with the TDBI and        p-well isolation in CMOS with STI. The width of this implant is        determined in one embodiment as about 0.5 um. The effect of this        implant starts to disappear as it becomes narrower. The        misalignment and control of the width also become more        difficult. If it is laid out wider, the maximum of the potential        becomes flatter and shifts towards the PD center.

FIG. 16 is a plan view schematic of a pixel, similar to FIG. 15. Thephotodiode area 143 is characterized by an n-type diode ring implant320. Cross-section 1600 for the 2D simulation is indicated.

-   -   Best dose and energy for the phosphorous ring implant is in the        range of 5e11 to 1e13 and 50 keV to 250 keV.    -   The optimum dose and energy for the Phosphorous implant is        determined by process and device simulations as explained in the        design methodology. Cross-sectional view is shown in FIG. 32.        The fabrication process is shown in FIGS. 17-31.

In FIG. 17, shallow trench isolation 702 and SiO₂ 701 are formed onp-type substrate 700. FIG. 18 shows the patterning 703 and implantation704 of the transfer device barrier implant. FIG. 19 shows the transferdevice barrier implant 705.

FIG. 20 shows the patterning 706 and implantation 707 of a typicalp-well implant. FIG. 21 shows the p-well implant 708 for the signal andreset transistors and isolation 709 of neighboring photodiodes.

FIG. 22 shows the patterning 710 and implantation 711 of the photodiodering implant. FIG. 23 shows the photodiode ring implant 712.

FIG. 24 shows the patterning 713 and implantation 714 of the photodiodedeep implant. FIG. 25 shows the photodiode deep implant 715.

FIG. 26 shows the patterning and growth of the gate oxide and patterningand deposition of the polysilicon gates 716 and 717.

FIG. 27 shows the patterning 718 and implantation 719 of the nldd, orfloating node. FIG. 28 shows the patterning 721 and implantation 722 ofthe pldd, or pinning implant. FIG. 29 shows annealing (The nldd or n+floating diffusion 720 and the pldd or p+ pinning layer 723 are shown)and the growth of the transistor spacers 724.

FIG. 30 shows the patterning 725 and implantation 726 of the sources anddrains of the transistors. FIG. 31 shows the annealing.

4) Work Function Control of Transfer Device Gate

The barrier of the transfer gate is optimized by using a polycrystallinesilicon gate with a more p-type work function, resulting in improvedproperties. This increase in barrier properties allows improved overallperformance when combined with a voltage boost on the transfer gateduring the on state. The higher work function makes the off state more“OFF” without the use of higher doping levels and attracts holes to thesilicon surface under the TG (pinning), reducing dark current viaelectron-hole recombination. The increased barrier can then be easilyovercome by a controlled voltage applied to the transfer gate during theon or charge transfer state.

There is also a significant reduction of the dark current by using apinned PD rather than a normal pn diode. The pinned PD keeps the surfaceaccumulated by holes, and therefore any electron that becomes free dueto surface interface states recombines with the hole immediately, andthe dark current is eliminated. The silicon under the TG area with theexisting structure remains depleted during operation. This area stillcontributes to dark current generation which can be eliminated bychanging the poly doping to p-type.

The TG in some embodiments uses an n-type poly. With typical operatingvoltages of the CMOS process, the area under the TG remains depleted andgenerates dark current.

There are other solutions to this problem such as applying a negativevoltage to the TG and attracting holes to the surface while the gate isOFF. This requires additional negative bias voltage which is morecomplex to create and manage.

Using p-type poly gate instead of n-type poly gate for the TG: Thework-function difference between the p-type poly to the substrate actslike negative biasing and attracts holes to the surface. By making theTG polycrystalline silicon p-type, the dark current generation under theTG is eliminated during the charge collection period.

The work function of the gate can also be sloped to be non-constant withthe work function over the n-type area being more p-type than the workfunction over/near the sense node.

-   -   A charge transfer device with a more p-type work function is        achieved by: blocking n-type doping normally applied to NMOS        type devices, and/or applying p-type doping which is also used        for surface pinning, and/or adding additional p-type implants        such as a special o-type implant, PLDD or p+ implant.    -   One embodiment for small pixels dopes the poly p-type with a        special mask after polycrystalline silicon deposition and before        patterning. Suitable polycrystalline silicon etch should be        obtained with the p-type doping present in the CMOS process        baseline.

FIG. 33 illustrates the p-type poly TG 717 and the silicon under thegate while it is on OFF state. The charge integration period is usuallymuch longer than the charge transfer period. Thus, pinning the siliconunder TG will eliminate the dark current generation in this regionalmost completely. This region will be in depletion only for a shortperiod of time during charge transfer. The dark current generationduring this time is negligible.

5) Adaptive Circuitry—Blooming Control—Incomplete Charge Transfer

Blooming Control during charge integration: Because the transfer gate'svoltage is variable, it can be used for blooming control during chargeintegration. If there is excessively bright light, there will be excesscharge in the PD. This excess charge should rather be drained to thefloating diffusion node of the same pixel rather than cause blooming tooccur in the neighboring pixels. If the light level is so high thatthere will be blooming in the adjacent pixels, the TG voltage should belowered, to enable the excess charge to flow to the floating diffusionnode easily. This voltage is variable and can be adjusted to the desiredlevel by the on-chip adaptive circuitry. (FIG. 5).

Charge Transfer: The same gate voltage is used differently, in the caseof incomplete charge transfer from the PD to the floating diffusionduring the charge transfer period. The TG voltage should be increased byan on-chip charge pump. The charge pump provides voltages at least athreshold voltage of the n-type MOS transistor (VT) above the supplyvoltage or higher. This gate operates under a strong backbias condition.Therefore, it can handle relatively high voltages. This feature providesextra margin for the charge transfer. (FIG. 8).

6) Optimization of Starting Material

Optimum integration of the advanced pixel occurs when the CMOS startingmaterial is chosen to be low doped p on p+ epi. The use of p+ epieliminates latch up concerns that might otherwise arise from anysignificant change in the substrate concentration.

The doping level should be as low as practical with acceptable controlin the manufacturing process and is recommended to be in the range of2E14 to a maximum of 1E15. This low doping allows the best optimizationof the charge transfer and allows the electric fields for chargecollection to extend as deeply as possible into the silicon. Oneembodiment has about 4E14 Boron doping.

The thickness of the lightly doped surface layer should be optimized toallow the best possible light collection. The choice of a thick epilayer improves light collection for the red but increases cross talk andincreases the potential for latch up. Use of a thin layer will interferewith CMOS n-well and p-well doping and results in reduced lightcollection in the red.

The optimum EPI thickness is in the range of 4 to 7 microns. (FIG. 34).

FIG. 34 is a cross-sectional schematic of a photodetector of a pixel.The p+ epitaxial layer having a low concentration layer 530 having athickness 520 is formed on p+ layer of substrate 510. The N-typephotodiode 550 is formed in the layer 530.

7) Pixel Layout

Contacts to polysilicon that are over the active channel are shown inFIG. 35. The placement of contacts over the pixel channel allows thepixel layout to be smaller with a higher fill factor and betterperformance.

8) Dark Current Reduction through Use of PLDD Implant to Isolate thePhotodiode from the Surface States.

The use of the PLDD implant as the p-type surface pinning implant savesone mask and works well. This implant pins the surface to the mostnegative voltage in the device and keeps the surface accumulated byholes. In addition, as explained above, the region under the TG can bepinned by doping this gate p-type. This region is depleted only duringcharge transfer period, which is typically much shorter than the chargeintegration period. Thus, the PD surface is pinned, and the dark currentgeneration is eliminated.

9) Correlated Double Sampling (CDS) and Very Low Noise Improve Low-LightPerformance.

Due to surface pinning in the PD, the dark current shot noise becomesvery low. The other major noise component is the kTC noise in the pixel.With the 3 transistor pixels, a true CDS cannot be done. Embodiments ofthe pixel design enable a true CDS by holding the charge isolated in thePD region. The FD potential is sampled twice, before and after thecharge transfer. The difference of these levels is due to the signalintegrated in the PD. Subtracting the signal level from reset leveleliminates the kTC noise.

As the noise level is reduced the low-light performance of the imager isimproved. This is very important for the digital still cameraapplications.

10) Shared Pixel Architecture

The shared pixel schematic is shown in FIG. 2, and the layout in FIG.35. This pixel works fine in a single pixel architecture. However, it isalso very suitable for a shared architecture because the transfer gatescan be laid out very symmetrically and be surrounded by the TDBI, whilethe reset gate, source-follower and row-select transistors are laid outin the p-well next to the photodiodes. It becomes also very simple tolay out the deep PD n-implant and p+—pinning implant to cover all 4 PDsat once.

When the charge integration period is over, the TGs turn ON one by one,transferring the charge from the PD to the floating diffusion node. Thecharge transfer occurs in the vertical direction for all the PDs and isvery symmetrical. The TGs are laid out as very simple gate structures toavoid 3D effects, especially caused by comers. The accumulated chargedoes not need to turn any comers and change direction with this layout,and flows vertically in one dimension.

The four PDs in the shared architecture are surrounded by the TDBI forisolation. The TDBI is also used as a barrier layer between them. Theactive devices (Reset transistor, Source Follower and Row-selecttransistors) are laid out in the p-well.

The layout becomes more efficient by

-   -   putting all the PDs together and isolating them with the TDBI,        and    -   sharing the active transistors that are laid out in the p-well.

About 30% . . . 40% higher fill factor is achieved compared to singlepixel architecture.

FIG. 36 is a simplified block diagram of a computer system 3610 suitablefor use with embodiments of the present invention. Computer system 3610typically includes at least one processor 3614 which communicates with anumber of peripheral devices via bus subsystem 3612. These peripheraldevices may include a storage subsystem 3624, comprising a memorysubsystem 3626 and a file storage subsystem 3628, user interface inputdevices 3622, user interface output devices 3620, and a networkinterface subsystem 3616. The input and output devices allow userinteraction with computer system 3610. Network interface subsystem 3616provides an interface to outside networks, including an interface tocommunication network 3618, and is coupled via communication network3618 to corresponding interface devices in other computer systems.Communication network 3618 may comprise many interconnected computersystems and communication links. These communication links may bewireline links, optical links, wireless links, or any other mechanismsfor communication of information. While in one embodiment, communicationnetwork 3618 is the Internet, in other embodiments, communicationnetwork 3618 may be any suitable computer network.

User interface input devices 3622 may include a keyboard, pointingdevices such as a mouse, trackball, touchpad, or graphics tablet, ascanner, a touchscreen incorporated into the display, audio inputdevices such as voice recognition systems, microphones, and other typesof input devices. In general, use of the term “input device” is intendedto include all possible types of devices and ways to input informationinto computer system 3610 or onto computer network 3618.

User interface output devices 3620 may include a display subsystem, aprinter, a fax machine, or non-visual displays such as audio outputdevices. The display subsystem may include a cathode ray tube (CRT), aflat-panel device such as a liquid crystal display (LCD), a projectiondevice, or some other mechanism for creating a visible image. Thedisplay subsystem may also provide non-visual display such as via audiooutput devices. In general, use of the term “output device” is intendedto include all possible types of devices and ways to output informationfrom computer system 3610 to the user or to another machine or computersystem.

Storage subsystem 3624 stores the basic programming and data constructsthat provide the functionality of certain embodiments of the presentinvention. For example, the various modules implementing thefunctionality of certain embodiments of the invention may be stored instorage subsystem 3624. These software modules are generally executed byprocessor 3614.

Memory subsystem 3626 typically includes a number of memories includinga main random access memory (RAM) 3630 for storage of instructions anddata during program execution and a read only memory (ROM) 3632 in whichfixed instructions are stored. File storage subsystem 3628 providespersistent storage for program and data files, and may include a harddisk drive, a floppy disk drive along with associated removable media, aCD-ROM drive, an optical drive, or removable media cartridges. Thedatabases and modules implementing the functionality of certainembodiments of the invention may be stored by file storage subsystem3628.

Bus subsystem 3612 provides a mechanism for letting the variouscomponents and subsystems of computer system 3610 communicate with eachother as intended. Although bus subsystem 3612 is shown schematically asa single bus, alternative embodiments of the bus subsystem may usemultiple busses.

Computer program medium 3640 can be a medium associated with filestorage subsystem 3628, and/or with network interface 3616. The computerprogram medium can be an optical, magnetic, and/or electric medium thatstores circuit data such as a layout, a tapeout, or other design data.

Computer system 3610 itself can be of varying types including a personalcomputer, a portable computer, a workstation, a computer terminal, anetwork computer, a television, a mainframe, or any other dataprocessing system or user device. Due to the ever-changing nature ofcomputers and networks, the description of computer system 3610 depictedin FIG. 36 is intended only as a specific example for purposes ofillustrating the preferred embodiments of the present invention. Manyother configurations of computer system 3610 are possible having more orless components than the computer system depicted in FIG. 36.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

1. A method of fabricating an image sensor integrated circuit having aplurality of photodetectors using energy of photons reaching theplurality of photodetectors to excite electrons; a plurality of nodes,each of the plurality of photodetectors having a corresponding node ofthe plurality of nodes; a plurality of transfer devices each with afirst terminal coupled to one of the plurality of photodetectors, asecond terminal coupled to the corresponding node, and a controlterminal causing a transfer of the electrons from the first terminal tothe second terminal in response to receiving a control signal ofsufficient value; a plurality of reset devices, each of the plurality ofnodes having a corresponding reset device of the plurality of resetdevices, and said each of the plurality of nodes is reset when thecorresponding reset device is active; and a plurality of signal devicescoupling the plurality of nodes to row and column circuitry; the methodcomprising: forming a plurality of oxide structures shaped to isolateadjacent photodetectors from each other; implanting a first n-typeregion for each of the plurality of photodetectors, the first n-typeregion receiving the electrons excited by the energy of the photons, thefirst n-type region at a first depth range in the integrated circuit;implanting a second n-type region for each of the plurality ofphotodetectors, the second n-type region at a second depth range whereinthe first depth range includes depths deeper than the second depthrange, adjacent to the first n-type region and at least part of thesecond n-type region positioned between the first n-type region and anoxide structure of the plurality of oxide structures, and receiving theelectrons from the first n-type region; depositing the control terminalfor each of the plurality of transfer devices at least partly above thesecond n-type region of the plurality of photodetectors; implanting theplurality of nodes, wherein each of the plurality of photodetectors hasa corresponding node of the plurality of nodes where the electrons aremeasured prior to removal; and implanting at least one n+ terminal foreach of the plurality of transfer devices, wherein said method forms rowand column circuitry on the image sensor integrated circuit accessingthe plurality of nodes.
 2. The method of claim 1, further comprising:implanting a plurality of p-type regions isolating neighboringphotodetectors from each other.
 3. The method of claim 1, furthercomprising: implanting a plurality of p-type regions on the plurality ofoxide structures.
 4. The method of claim 1, further comprising:implanting a plurality of p-type regions in which the plurality of resetdevices, and the plurality of signal devices are formed.
 5. The methodof claim 1, wherein said depositing the control terminal includesdepositing the control terminal for the plurality of reset devices andthe plurality of signal devices.
 6. The method of claim 1, wherein saidimplanting at least one n+ terminal includes implanting the n+ terminalsof the plurality of signal devices and the plurality of reset devices.7. The method of claim 1, wherein a difference in n-type concentrationbetween the first n-type region and the second n-type region causeselectrons to move from the first n-type region to the second n-typeregion.
 8. The method of claim 1, wherein the second n-type regionsurrounds a perimeter of the first n-type region.
 9. The method of claim1, wherein the second n-type region surrounds a perimeter of the firstn-type region, and the second n-type region is formed with phosphorousimplanted in an energy range of 80 to 120 keV and in a dose range of1×10¹² to 6×10¹² atoms/cm².
 10. The method of claim 1, wherein thesecond n-type region surrounds a perimeter of the first n-type region,and the first n-type region is formed with phosphorous implanted in anenergy range of 280 to 320 kev and in a dose range of 8×10¹¹ to 1.4×10¹²atoms/cm².
 11. The method of claim 1, further comprising: a p-typeregion surrounding the first and second n-type regions, and a p-typedoping of the p-type region has a higher concentration than n-typedoping of the first and second n-type regions.
 12. The method of claim1, further comprising: a p-type region surrounding the first and secondn-type regions, and a p-type doping of the p-type region has a higherconcentration than n-type doping of the first and second n-type regions,wherein the higher concentration assists depletion of the second n-typeregion.
 13. The method of claim 1, further comprising: a p-type regionsurrounding the first and second n-type regions, and a p-type doping ofthe p-type region has a higher concentration than n-type doping of thefirst and second n-type regions, wherein the p-type region isolates thefirst and second n-type regions from adjacent photodetectors.
 14. Themethod of claim 1, wherein each of the plurality of transfer devicesincludes a body connecting the first terminal and the second terminalsuch that the control terminal controls the transfer of the electronsbetween the first terminal and the second terminal through the body, anda dielectric between the control terminal and the body, the dielectricsatisfying a lifetime specification of the image sensor integratedcircuit when the control signal is applied with the channel formed, thedielectric failing the lifetime specification of the image sensorintegrated circuit if the control signal is applied with at least one ofthe first terminal and the second terminal at a ground voltage of theimage sensor integrated circuit.
 15. The method of claim 1, wherein theplurality of signal devices includes a plurality of row selecttransistors coupled to the row and column circuitry and a plurality ofsource follower transistors coupled to the plurality of nodes.
 16. Themethod of claim 1, wherein the plurality of photodetectors is aplurality of photodiodes.
 17. The method of claim 1, wherein eachmeasurement of the total of the photons is corrected by correlatedmultiple sampling with a prior measurement of the total of the photons.18. The method of claim 1, wherein the plurality of oxide structuresincludes a plurality of pn junctions helping to isolate adjacentphotodetectors from each other.